The present invention pertains to a process for the cross-linking of modified engineering thermoplastics, in particular, of polymeric sulfinic acids or sulfinic acid salts.
Various ways for preparing cross-linked polymers and membranes are known in the prior art. Some of these may be mentioned here:
1) Preparation of cross-linked membranes by the copolymerization of mono- and bifunctional monomers (example: copolymerization of styrene and divinylbenzene in thin layers, followed by sulfonation of the cross-linked polymer membrane produced). This method has the disadvantage that the cross-linked ionomer membranes produced thereby have limited oxidation stabilities since both styrene and divinylbenzene contain tertiary Cxe2x80x94H bonds which are sensitive to oxidation. (For a comparison of the oxidation sensitivities of styrene having a tertiary Cxe2x80x94H bond and of xcex1-methylstyrene without a tertiary Cxe2x80x94H bond, see, for example: Assink, R. A.; Arnold C.; Hollandsworth, R. P., J. Memb. Sci. 56, 143-151 (1993).)
2) Functionalization of ion-exchanging groups of ionomers, followed by reaction with bi- or oligofunctional cross-linking agents to yield a cross-linked membrane (example: functionalization of sulfonic acid groups to yield the reactive acid chloride or imidazolide, followed by reaction with (aromatic) amines; Nolte, R.; Ledjeff, K.; Bauer, M.; Mxc3xcllhaupt, R.; J. Memb. Sci. 83, 211 (1993)). In these cross-linking methods, cross-links (sulfonamide groups) are formed in the membrane which are not sufficiently stable to hydrolysis.
3) Chemical activation of ion-exchanging groups of ionomers, followed by reaction of the activated group with other groups of the polymer main chain to yield a cross-linked membrane (example: conversion of part of the sulfonic acid groups of sulfolated PEEK (polyetheretherketone) to sulfochloride groups, attack of the sulfochloride groups during membrane formation in the hydroquinone region of the PEEK repeating unit under Eriedel-Crafts acylation and formation of hydrolysis-stable xe2x80x94SO2xe2x80x94 links (EP 0 574 791 A2). This method has the disadvantage that it can be employed only with certain aryl polymers, such as PEEK.
From Kice, J. L.; Pawlowski, N. E: J. Org. Chem 28, 1162 (1963), it has been known that low-molecular sulfinic acids can disproportionate according to the following scheme of reactions; 
From Quaedvlieg, M., in: Houben-Weyl, Methoden der Organischen Chemie, Vol. II, 606, Thieme Verlag, Stuttgart (1957); Ashworth, M. R. F., in: The Chemistry of Sulphinic Acids, Esters and their Derivatives (Ed.: S. Patai), chapter 4, 97-98, John Wiley Ltd., New York (1990); Allen, P.: J. Org. Chem. 7, 23 (1942), it has been known that sulfinate can readily be alkylated to the sulfonate. The reaction was performed, inter alia, in alcohols having different chain lengths: 
It has, been the object of the present invention to provide novel processes for cross-linking modified engineering thermoplastics, in particular, of polymeric sulfinic acids or salts thereof.
The above object is achieved, in a first embodiment, by a process for the preparation of cross-linked polymers, characterized in that solutions of polymeric sulfinic acids or sulfinic acid salts (xe2x80x94SO2Me), optionally in the presence of organic di- or oligohalogeno compounds [R(Hal)x], are liberated from solvent and cross-linked to polymers,
wherein Me stands for a monovalent or polyvalent metal cation;
R stands for an optionally substituted alkyl or aryl residue containing from 1 to 20 carbon atoms; and
Hal stands for F, Cl, Br or I.
It is particularly preferred that the sulfinic acids or salts thereof is derived from structures comprising aromatic nuclei having R1 or R2 structures of the following formulae as the repeating unit wherein 
R3 stands for hydrogen, trifluoromethyl or CnH2n+1, with n being from 1 to 10, in particular methyl;
R4 stands for hydrogen, CnH2n+1, with n being from 1 to 10, in particular methyl or phenyl; and
x stands for 1, 2 and 3,
which are linked through bridging groups R5 or R6 wherein
R5 stands for xe2x80x94Oxe2x80x94, and
R6 stands for xe2x80x94SO2xe2x80x94.
It is particularly preferred according to the present invention that the solvent is selected from dipolar-aprotic solvents, such as NMP, DMAc, DMSO or DMF.
The polymer or one of the blend components is a polymer modified with sulfinic acid groups (xe2x80x94SO2H) and/or sulfinic acid salt groups (xe2x80x94SO2Me) (Me=Li, Na, K, Rb, Cs or other mono- or di-valent metal cations). The choice of basic materials (polymeric sulfinic acids/sulfinic acid salts) is not limited; all polymeric or oligomeric sulfinic acids can be employed as the basic materials. The advantage of the cross-linking method according to the invention over the prior art is characterized by the following items:
The processes can be universally employed: all polymeric sulfinates and sulfinic acids can be cross-linked according to this process.
A wide variety of polymers can be blended with the polymeric sulfinic acids/sulfinic acid salts. Cross-linked polymer blends are obtained having blend morphologies (microstructures) and cross-linking densities which can be adjusted within wide ranges.
The cross-linked polymers have improved thermal and chemical resistance as compared with the basic substances.
The hydrolytic stabilities of the cross-linked polymers and membranes are substantially improved over the hydrolytic stabilities of other cross-linkings, for example, the cross-linking of polymeric sulfonates through sulfonamide links.
It has now been surprisingly found that the per se known disproportionation reaction can be made use of for cross-linking polymers by preparing sulfinic acid containing polymers according to a known method (Kerres, J.; Cui, W.; Neubrand, W.; Springer, S.: Reichle, S.; Striege, B.; Bigenberger, C.; Schnurnberger, W.; Severs, D.; Wagner, N.; Bolwin, K.: lecture (lector: J. Kerres), Euromembrane ""95 Congress, Bath (UK), Sep. 18 to 20, 1995, Proceedings, pages 1-284 (1995)) or other methods and cross-linking the same via the above disproportionation reaction, wherein other polymers may be added to the solution of the polymeric sulfinic acid in an appropriate solvent (for example, DMSO, DMAc, DMF, NMP) which polymers are then integrated in the forming polymeric network.
The microstructure of the generated cross-linked polymer blend depends on the compatibility of the blend components. If all blend components are compatible with each other, an interpenetrating network will form in which the polymeric chains of the blend components are entangled to such an extent that the blend components can hardly be separated any more. If the blend components are immiscible with each other, a microphase structure will form in which some of the components are dispersed in a continuum of the other components. The microphase structure is dependent on the kind of the blend components and the mixing ratio of the blend components. By appropriately selecting the blend components and their mixing ratio, cross-linked blend structures can be produced as desired.
The cross-linking process according to the invention can be employed, for example, for the preparation of cross-linked cation-exchange membranes. There are two ways of proceeding:
A polymeric sulfinic acid and a polymeric sulfonic acid are dissolved together in the same solvent (DMSO, DMAc, DMF, NMP) and then further processed.
A polymer the sulfinic acid groups of which have partially been oxidized to sulfonic acid groups (oxidation level 0 to 100%; the oxidation level can be adjusted through the quantity of oxidant added to the polymeric sulfinate) is dissolved in the solvent and then further processed.
This polymer solution is then cast on a substrate (glass plate, polished aluminum sheet, woven or non-woven fabric). Thereafter, the plate is placed in a drying oven and the solvent evaporated at elevated temperature, in particular above the boiling point of the solvent under the pressure conditions chosen, During the evaporation process, cross-linking of the polymeric sulfinic acid occurs. It has now been surprisingly found that the generated cross-linked cation-exchange membranes have a significantly increased thermal resistance as compared with the non-cross-linked polymeric sulfonic acid. Such cross-linked cation exchange membranes can now be employed to advantage in electromembrane processes, for example, in electrodialysis or in membrane fuel cells. In particular, their use is of advantage in such cases where it is necessary to operate at elevated temperatures and under severe chemical conditions since the cation-exchange membranes cross-linked according to the process invention exhibit reduced swelling in addition to an increased thermal stability as compared with the swelling of non-cross-linked cation-exchange membranes having the same cation-exchange capacity.
It has been shown that the yield of sulfone formation increases when the chain length of the alcohol is increased from CH2xe2x80x94CH2 to CH2xe2x80x94CH2xe2x80x94CH2xe2x80x94CH2 [P. Allen, J. Org. Chem. 7, 23-30 (1942)]. It has now been surprisingly found in preliminary experiments that this reaction can be made use of for cross-linking polymers which contain sulfinate groups.
Surprisingly, this cross-linking process could be successfully used for cross-linking ion-exchange membranes as well. The basic conception had been as follows: The Li salt of the polymeric sulfinate, for example, polysulfone Udel(copyright) (PSU)-Li sulfinate which is obtained by the reaction of metallated PSU with SO2, is mixed with the Li salt of polymeric sulfonates, for example, PSU-Li sulfonate which is obtained by the oxidation of PSU-Li sulfinate with oxidants, or with the Li salt of sulfonated PEK and the mixture is dissolved in di-polar-aprotic solvents, such as NMP, DMAc, DMSO or DMF. Then, an organic di- or oligohalogeno compound in which the halogen atoms can be substituted by nucleophilic substitution is added to the solution which is then stirred until complete mixing occurs. Thereafter, a thin film of the solution is prepared on a substrate, for example, on a glass plate, and the solvent is evaporated at: elevated temperature under reduced pressure or by using a forced convection. During the evaporation of the solvent, the organic di- or oligohalogeno compound reacts with the sulfinate residues of the polymers under cross-linking to disulfone links and with formation of halogenide anions. The polymeric chains of the polymeric sulfonate are incorporated in the network of the polymeric sulfinate. The network prevents the sulfonate component of the polymer blend membrane from swelling too much and significantly increases the chemical stability of the cross-linked membrane. As in the other cross-linking process mentioned above, another possibility is to partially oxidize a polymeric sulfinate, for example, PSU(SO2Li), to the polymeric sulfonate (every oxidation level between 0 and 100% being possible) and then to react the partially oxidized polymer with the dihalogeno compound. The advantage of this process is that the whole polymer is involved in the cross-linking reaction, in contrast to the cross-linking of polymer blends where only the polymeric sulfinate component is cross-linked.